1. Field of the Invention
The present invention relates to improved diffuser material, and more particularly concerns high efficiency diffuser material that can be applied to optical systems requiring controlled diffusion.
2. Description of Related Art
The function of the projection screen of an optical projection display system is to present a visible display of imaging light that it receives, regardless of whether the light is produced by a cathode ray tube, a liquid crystal light valve, a laser, or other image source. The screen forms a visible image by diffusing and re-radiating light energy incident on the screen surface. The front projection screen includes a diffusion sheet comprising diffusion particles and a reflective rear face so that light striking the front of the screen is reflected and passes through the diffusing particles twice. The rear projection screen also includes a diffusion sheet carrying diffusion particles but receives light striking the rear of the screen, passes it through its diffusion particles only once, and transmits diffused light from its front face.
Whether used in a front projection or rear projection configuration, the screen has a major impact on image quality and viewability, which may be defined as the zone in which the image is of acceptable luminance. A high screen gain enhances viewability. Screen gain is the comparison of (a) the luminance of a projected image at various viewing angles, with (b) the luminance produced by a perfectly diffusing surface (e.g. a Lambertian surface). The ideal diffuser has a gain of unity, and produces the same luminance at all viewing angles, that is, it is non-directional. A Lambertian diffuser has the same luminance screen in all directions, which falls off as the cosine of viewing angle.
Projection systems employing such viewing screens are often made with highly directional screens to increase screen gain so as to produce sufficient luminance. However, because the screen itself cannot generate light, any increase in luminance at a particular viewing angle causes a decrease in luminance at other viewing angles. Nevertheless, because many systems are normally viewed only at a small viewing angle, or because a high gain small viewing angle is deemed to be worth a decreased illumination at large viewing angles, shaping of screen diffusion patterns may be desirable.
To provide a diffusing screen with increased screen gain the diffused light pattern has been shaped to change it from its normally circular format to an elliptical pattern. However, prior beam shaping materials and techniques are difficult and expensive to manufacture and use. For example, holographic diffusers have been employed having controllable micro-variations in refractive index of a volume hologram to achieve adjusted refractive index variations and selected scattering patterns. These holographic devices are limited in size.
Some front projection screens have been constructed utilizing mathematically designed micro-mirror contoured reflecting surfaces to catch and return projected light directionally in a precisely focused manner. Again, these arrangements are difficult to manufacture and exceedingly expensive.
Accordingly, it is an object of the present invention to provide a screen having improved controlled diffusion characteristics while avoiding or minimizing above mentioned problems.